1. Field of the Invention
The invention relates generally to radar devices and more particularly, to continuous-wave radar devices which utilize pseudo-random coded, bi-phase modulation.
2. Description of the Prior Art
In the radar system arts, the so-called pseudo-random coded (PRC) system is known both in continuous-wave (CW) radar systems and also in pulse radar. In pulsed radar a predetermined code is selected over a comparatively long transmission pulse, and then pulse compression is employed at a receiving station, which may be a remote location or at the same location as the transmitter in which case the useful received signals are normally those reflected from distant objects.
In CW pseudo-random systems, a much longer code word is employed, more correctly referred to as a maximal length code. Such codes are imposed on the radio frequency carrier of the transmitter as a series of discrete transmitter phase levels, usually zero and 180.degree. phases of the carrier signal between these two discrete levels and, accordingly, the modulation is referred to as bi-phase. When received and detected, these discrete bi-phase signals are readily converted to a corresponding series of "1" and "0" video levels which appear to occur randomly. Actually, however, they are recurrent after L bits and, therefore, are called pseudo-random codes (PRC).
The state of the art in respect to both pulsed and CW radar systems employing pseudo-random codes or sequences, as they are sometimes called, is extensively described in the technical literature. For example, the textbook, "Radar Handbook" by Merrill I. Skolnik (McGraw-Hill 1970), summarizes the subject and provides additional bibliographic references. The general methods of instrumenting the generation of pseudo-random sequences in the transmitting equipment of such a radar are described in Chapter 20 of that text. The so-called auto-correlation property of the maximal length pseudo-random sequence is also described.
Notwithstanding the advantages of CW radar, in respect to high average power on target and other matters, there are certain prior art problems associated with CW PRC radars which have tended to limit their usefulness and the realization of their inherent advantages. Phenomena such as "blind-speeds" and range and velocity ambiguities are among the phenomena limiting the usefulness of CW PRC in the prior art. The causes of such ambiguities are well understood, and the general subject is discussed in Chapter 3 of the aforementioned reference textbook.
Pseudo-random coded sequences are readily generated using digital techniques. A clock-pulse generator is arranged to feed a coder-shift register which is wired with the appropriate feedback. The resultant output of the coder is a series of "1" and "0" video levels which, as hereinabove indicated, appear to occur randomly. After L bits, the sequence repeats a code word of length L, related to the number of shift register stages N by the equation L = 2.sup.N - 1. Accordingly, a five-stage register results in a 31 bit code. This code in video form is then fed to an RF bi-phase modulator where it encodes the bit sequence onto a carrier in a corresponding sequence of 0 and .pi. phase, according to the PRC code. The resultant signal is wideband (approximately twice the code clock-rate) and contains the coding information.
The auto-correlation of the sequence is generated by delaying the code in time (or automatically delaying it in accordance with range in a radar system) with respect to the same code, multiplying the two codes, and then integrating. For maximal length pseudo-random codes, the resultant auto-correlation function always has the same shape, thus, when the codes are aligned in time (or range) i.e., bit 1 with bit 1, bit 2 with bit 2, etc., throughout the code, the integrator output is maximum and equal to L units of amplitude. When the displacement between the received and reference (local) codes is one or more bits, the output drops to a value of -1 and thus, the resolution achievable in time or range, is equivalent to that achieved in a conventional pulse system with a pulse-width equal to .tau..sub.b, the bit width.
In the prior art, various ways have been devised for dealing with one or the other of the inherent ambiguities in a PRC CW radar system, one of these being described in U.S. Pat. No. 3,641,573. In the device described in that patent, a tone is used so that a gross range value can be determined and used to resolve the ambiguities inherent in the range measurement with a limited length transmitted code less than the range of the received signal. In effect, in that device, the pseudo-random code provides a vernier range measurement.
A PRC radar system will generally improve overall performance as compared to CW radars of the FM type of 15 to 30 dB in the environments of spillover and clutter. Spillover represents a major problem with all CW type radars. Spillover, in this context, means the undesired direct energization of a receiver from the transmitter, and/or reflected close-in clutter. That particular problem is extant because of the fact that time-sharing is not possible as with pulsed radars.
By using PRC concepts in a radar system, a reduction in spill-over by 1/L.sup.2 (30dB for a 31 bit code) as compared with spillover signal levels encountered in unmodulated CW radar (all other factors considered to be constant). In the first (nearest) range bin or range increment, there may be no advantage over unmodulated CW radar, however, at longer ranges, the advantage is substantial.
In respect to "out-of-range clutter, response is down 30 dB in the 31 bit code PRC arrangement" (because of correlation rejection), below that of a CW unmodulated radar and is therefore negligible. In-range clutter does correlate in the detection process of a PRC system, but represents on the average only 1/L (1/31 in the case of the 31 bit code aforementioned) of the total clutter power reflected to the receiver of a CW radar. This constitutes an improvement on the order of 15dB.
The manner in which the device of the present invention provides unambiguous range measurement and also unambiguous Doppler determination will be understood as this description proceeds. Moreover, the advantages of the present system over other prior art approaches to the general problem will become apparent.